The present invention relates to a reading/writing pickup head, and more particularly to an optical pickup head for reading or writing data on optical record media such as compact disks or digital versatile disks.
It is known that optical pickup heads are used to record and reproduce information such as video or audio data on and from optical data recording media. One characteristic function of a pickup head is to shape a laser beam for efficiently reading and recording data on an optical medium. This function comes from two reasons: Firstly, Since it needs much power for a pick-up head to write data on a disk. The collimator should collect more power from the laser source in order to reduce the load of the laser source. Secondly, in general, the laser beam is produced by a laser diode, and the shape of the laser beam is originally elliptical. In order to increase the reading and writing quality of the pickup head, the laser beam is shaped to be circular. The other characteristic function of the pickup head is to split the incident and reflected light branches for reducing the interference of both light branches as the reading and record operation of the pickup head.
A pickup head of the related art is shown in FIG. 1. A prism 91 made of optical glass is placed in the incident branch of the pickup head, and it is used to shape the elliptical laser beam to a circular beam. In order to split the incident and reflected branches of the pickup head, a polarized beam splitter (PBS) 93 and a quarter-wave plate (xc2xc xcex plate) 94 are used. In order to achieve both characteristic functions of a pickup head mentioned above, a lot of optical elements are required in the related art.
The object of the invention is to provide a pickup head which consists of fewer optical elements than the related art without losing both characteristic functions of a pickup head. With the pickup head of the present invention, the reading and writing quality of optical record media is increased.
According to the present invention, a birefringent prism and a quarter-wave plate are placed in the light path of the pickup head. The birefringent prism is made of double-refraction material. The refraction index of a double refraction material depends on the angle between its optical axis and the polarization of the incident beam. In other words, the double-refraction material may h as two different refraction indices for two different polarized beams, for example, for beams with polarization parallel and perpendicular to the optical axis. The birefringent prism is used to shape the elliptical laser beam as the prism of the related art, and used to split the incident and reflected branches of the shaped laser beam as the polarized beam splitter of the related art where the incident beam and the reflected beam have different polarizations. The prism and polarized beam splitter of the related art are replaced with the birefringent prism in the pickup head of the present invention.